Thermal evolution of ferrite in electrodeposited iron‑carbon coatings in relation to phase transformations during post-deposition annealing

The environmentally friendly electrodeposition of iron‑carbon coatings with excellent mechanical properties is a promising alternative for the banned hazardous hard chrome coatings, but the internal structure of the electrodeposits is still a matter of debate. The analysis of the microstructure and phase constitution is challenging for the nanocrystalline coatings, which contain in addition to an essential amount of carbon also oxygen and hydrogen, and an unambiguous phase identification of as-deposited coatings has not yet been obtained. In the present work, the evolution of the crystallite size and the microstrain in ferrite is followed in real time by in-situ annealing studies with energy-dispersive synchrotron diffraction. Ferrite is of particular interest, because it remains the major phase in the coatings, while other phase transformations occur during annealing. The thermal stability of the internal structure of ferrite, obtained as an indirect measure from the in-situ diffraction analysis during isochronal annealing, was verified with ex-situ transmission electron microscopy for selected samples. The results not only reveal the thermal evolution of the coating, but the correlation of the detected temperature-dependent changes of ferrite with the transformation of other phases than ferrite further supports the understanding of the as-deposited nature of the iron‑carbon coatings.